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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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472                                 ALASKAN GLACIER STUDIES
The ice in this northern portion of the channel was supplied by the expanded glaciers of College and Harriman Fiords, Passage Canal, Blackstone Bay, and Port Nellie Juan. It seems probable that at the TrunrinmTn stage, Esther, Culross and adjacent smaller islands were completely overridden by the glacier.
The second portion of this channel is the deep basin north of Knight Island. It is at least twelve miles long and 000 to 2400 feet deep, including not less than thirty square miles with a depth of 2400 feet. South of this basin the depth decreases to less than 1200 feet, suggesting that the depth of 2400 feet is due to glacial scooping caused by the obstruction to free outflow of the ice through the narrower channels to the southward.
Fiords and Islands of Western Prince William Sound. The fiord which connects directly with this submarine channel is northern Knight Island Passage. It is twenty-two miles long and 3 to 4 miles wide, with a depth of 800 to 2000 feet. When the glacier rose at least 400 feet above the crest of Chenega Island on the west and to a height of 2400 feet1 on Knight Island to the east, the depth of moving ice in this channel was at least 8500 to 4500 feet. We have little hesitancy in ascribing tremendous erosive power to this ice stream and believe that it produced the whole fiord, which has steeply-sloping walls and receives tributary bays like Drier Bay, which hangs over 1000 feet and contains submerged hanging valleys of the second order like Cathead and Mallard Bays which hang 214 and 264 feet respectively.2
The eastern border of tliis fiord on Knight Island shows much influence, both of local glaciers and of the ice which formerly filled the fiord. At the time of maximum glacia-tion in Prince William Sound, a few of the higher peaks of Knight Island probably rose above the ice sheet as a group of nunataks, for Grant and Higgins state1 that the island does not seem to have been glaciated higher than 2400 feet. The northern end of the island has rounded summits, but further south these give place to sharp peaks, with talus slopes and cirques containing snow, some of which may have been sharpened by the action of the local glaciers and by super-glacial weathering, perhaps after having been rounded by the erosion of the Knight Island Passage ice stream. Glacial erosion has been effective in rounding the predominating greenstones of Knight Island and producing a more knobby topography than on the slate and graywacke which make up Chenega Island, the mountains of Icy Bay, and other parts of the mainland to the west. There are U-shaped troughs and well-developed cirques and hanging valleys on Knight Island.
Within Knight Island Passage the shallowest portion of mid-channel, 800 to 1200 feet, is at the northern end of Chenega Island where a glacial distributary formerly moved southward through Dangerous Passage, west of Chenega Island. The deepest portion, over 2000 feet, is southeast of Chenega Island where a smaller island and the incoming of the expanded glacier of Icy Bay confined the ice tongue of Knight Island Passage to a narrow channel, forcing it to flow rapidly and erode deeply.
The southern portion of Knight Island Passage extends southeastward from Chenega Island to Montague Strait near Latouche Island. It is a fiord 4 to 6 miles wide and 15 miles long, exclusive of the mouth of Icy Bay. The depth ranges from 800 to 1200 feet.
i Giant, U. S. and Higgins, D. F., Bull. 448, U. S. Geol. Survey, 1910, p. 19. * Charts 8550, 8515, and 8524, U. S. Coast and Geod. Surrey.